An organic light emitting device is an electronic device in which a thin film containing a fluorescent organic compound or a phosphorescent organic compound is interposed between an anode and a cathode. In the organic light emitting device, holes and electrons are injected from each electrode to the thin film containing a fluorescent organic compound or a phosphorescent organic compound. When the holes and electrons are recombined in the thin film, excitons of the fluorescent compound or the phosphorescent compound are generated. When the excitons return to a ground state, the organic light emitting device emits light.
The recent advance in the organic light emitting device is remarkable, and the organic light emitting device can serve as a light emitting device with high luminance at a low applied voltage, various emission wavelengths, high-speed responsiveness, small thickness, and light weight. This suggests that the organic light emitting device may be used for a wide range of applications such as a flat panel display, a flexible display, and illumination.
However, when the organic light emitting device is driven continuously, constituent materials for the organic light emitting device are degraded with time, for example, by being combined with electrons or holes, and hence, the emission intensity of the organic light emitting device decreases with time.
The research and development for alleviating the decrease with time in characteristics such as emission intensity of the organic light emitting device during continuous driving have been conducted conventionally. Japanese Patent Application Laid-Open No. 2000-068064 discloses an organic light emitting device in which a hole transport layer is formed of at least two kinds of organic materials so as to enhance the durability of the organic light emitting device. Specifically, the patent document discloses an embodiment in which the hole transport layer contains at least two kinds of organic materials that are different in durability. This can prevent the crystallization and the growth of dark spots in an organic compound layer, particularly, the hole transport layer, and hence, the durability of the device is enhanced.
Meanwhile, Science, 283, 1900 (1999) mentions that an organic compound forming the organic light emitting device repeatedly undergoes oxidation and reduction caused by the current conduction through the device to decrease the emission intensity of the device. The above document also discusses that the decrease in emission intensity of the device is caused when a particular organic compound is oxidized, that is, enters a radical cation state. Specifically, the document suggests that, when the device is driven, tris(8-quinolinolato)aluminum (Alq3) incorporates holes to enter a radical cation state, which is one cause of the decrease in emission intensity of the device.
Considering the above-mentioned technical background, the inventors of the present invention discussed that an organic compound contained in a hole transport layer is involved in the decrease with time in emission intensity of the organic light emitting device as one cause. The hole transport layer is supplied with holes that are charge carriers from an anode. Therefore, the prevention of the structural change in the organic compound contained in the hole transport layer is important from the viewpoint of alleviating the decrease with time in characteristics such as emission intensity of the organic light emitting device during continuous driving.